The present invention relates to a novel naphthalocyanine dye, a method for preparing the same, and an optical information recording medium employing the same.
An optical information recording medium has an excellent characteristic feature that no wearing out or deterioration will not occur since the medium is used without coming into contact with a recording or a reading out head, and therefore developing researches have been made on various kinds of recording mediums. Particularly in the field of heat mode recording systems utilizing laser diode and the like, low-melting metals, organic polymers and dyes have been proposed as materials capable of melting, evaporating and sublimating. Above all, an organic film containing an organic polymer or a dye is preferable from view of high recording sensitivity, owing to a low heat conductivity and a low melting or sublimating temperature. As examples of such dyes, various materials such as cyanine dyes and squalirium dyes have been proposed.
Heretofore, the optical information recording medium in which the dye is used as a recording layer has been known, as disclosed in Japanese Patent Provisional Publication No. 16948/1981. However, if it is attempted to utilize this kind of dye film recording layer as a reflection type optical information recording medium, a metallic reflective film will be additionally required, which makes the medium constitution complicated and leads to the deterioration in information recording/reading out properties disadvantageously. In order to overcome this problem, it has been proposed to make use of a cyanine type dye or the like having a suitably high reflectance, as disclosed in Japanese Patent Provisional Publication No. 78787/1985. However, the cyanine type dye generally is poor in stability against light, and therefore it is liable to be decolorized by repeated irradiation of a reading beam at the time of reading after writing, with the result that an S/N ratio for the reading decreases inconveniently.
On the other hand, Japanese Patent Provisional Publication Nos. 25886/1986 and 177287/1986 propose to employ to the recording layer a naphthalocyanine compound as the dye which is excellent in light fastness. With regard to the naphthalocyanine dye disclosed in the former literature, its reflectance is usually lower as compared with the cyanine dye, and for this reason, its recording/reading out properties are poor and unsatisfactory. Further, in the latter literature, it is proposed to apply a material similar to the naphthalocyanine dye of the present invention to the information recording medium, but no examples for exhibiting effectiveness as the optical information recording medium have not been disclosed at all. Therefore, such a publication does not enable a person skilled in the art to judge whether the recording/reading out properties are excellent or not, or the level of reflectance is fully satisfactory or not.
In view of such a situation, intensive researches have been conducted to eliminate the above-mentioned drawbacks of the conventional techniques. As a result, it has been found that a novel naphthalocyanine dye synthesized by the present inventors has a reflectance equal to or higher than that of a cyanine type dye and is excellent in reading stability, when used to form an optical information recording medium, to accomplish the present invention.